Patent GB-874316-A (GB)
Title: Improved recording and/or reproducing system
IPC: H04N5/782
Filed: 1959-07-14
Abstract: 874,316. Automatic speed-control systems. AMPEX CORPORATION. July 14, 1959 [Aug. 8, 1958], No. 23231/59. Class 38 (4). Magnetic tape recording and reproducing apparatus suitable for colour television signals comprises a tape 11 which is driven past a transducing head assembly by a capstan 13, a plurality of heads 16 being carried on disc 18 driven by synchronous motor 19. A guide 21 serves to cup the tape as it is drawn past the transducing units, the tape being in continuous pressure contact with the heads as they sweep through a circular path. During recording, the velocity of the disc 18 is maintained in a predetermined relationship with that of the capstan 13, the same relationship being substantially maintained during reproduction by means of a control signal recorded on the tape by a head 31. A reference frequency may be derived from a video input signal through a sync separator 71, or from a local line through pulse former 73. Timing pulses, selected by switch 76, are applied to a multivibrator 77 forming square waves which are compared with the phase of a signal from divider 79. The output of divider 79 corresponds to the output of a photo-cell 53 which generates a square wave having a frequency dependent upon the speed of the disc 16. If there is a phase difference, a signal is fed to oscillator 80 which varies its frequency of operation, the output of the oscillator being applied through multivibrator 81 and saw-tooth generator 82 to mixer 83. The shaped photo-cell output is also applied to oscillator 84 producing damped sinusoidal oscillations in phase with the input pulses, a phase comparator 86 comparing a direct signal from the shaper 57 with that derived from the oscillator. Thus, the phase of the direct pulse is compared with that of the preceding pulse from the photo-cell, and any variations in head velocity during this interval generates an error signal which is applied to mixer 83. The error signal varies the level of the saw-tooth voltage applied to the mixer, this voltage being fed to a slicer 88 so that the phase of the trailing edge of the sliced signal is also varied. A multivibrator 89 generates a square wave having a frequency and phase dependent upon the difference between the photo-cell output and the reference frequency, and also dependent upon changes in head velocity between successive pulses. The output of the multivibrator 89 is filtered and amplified for driving the motor 19. During the recording operation, the output of divider 79 is applied through filter 98 to an amplifier 99 for driving a capstan motor 101. Thus the speed of the capstan motor is directly related to that of the disc 18. The output from the shaper 57 is also applied through amplifier 102 to the head 31 for forming a record track. During reproduction, the control track head is connected to amplifier 103 the signal from which is compared in phase with a signal from the photo-cell. The error signal is applied to an oscillator whose frequency is varied thereby, the modified signal being fed to amplifier 99 for driving the capstan motor 101. In a more detailed arrangement, the output video signal is applied to amplifier 106 and then through sync separators 107, 108 to an integrator 109 wherein pulses having video synchronizing frequency are formed. If the input is derived from a local power source, it is connected to a pip former 111 and pulse separator 112. The output of amplifier 114 is passed through multivibrator 116 to a phasesplitter 118 having two square-wave output signals phase-displaced by 180 degrees. These signals are applied to a phase comparator 119. The output of the photo-cell 53 is applied to an amplifier and clipper 120, thence to amplifier 121 and shaper 122. Each of the dividers 123, 124 divides the frequency by two, a phase splitter 125 forming a pair of square waves phase-displaced by 180 degrees. These waves are applied to comparator 119 wherein the signals from the phase splitters 118, 125 are compared. The error signal is filtered and its D.C. component is clamped. The clamped signal is fed to a stage controlling the reactance of the tuned circuit of oscillator 136, rectifiers 138, 139 determining the amount of signal applied to control circuit 141 whereby sudden changes of frequency are prevented. The output of the oscillator is fed through multivibrator 143 and saw-tooth generator 144 to mixer 146 having voltageregulating tube 147. A second signal is applied to the mixer from filter 148 which derives its input from the photo-cell. The output from shaper 122 is differentiated and applied to oscillator 149 providing a phase-shifted signal through a slicer circuit and amplifier to phase splitter 153. A phase comparator 154 compares this signal with one directly from the shaper 122, any change in head velocity providing an error signal to filter 148. The sliced and amplified output of the mixer is applied to the head motor amplifier 92. Thus, one servo loop synchronizes and positions the heads with respect to the reference frequency whilst another damps out oscillations of the disc 18 due to mechanical or electrical disturbances. In a further arrangement, gating pulses are obtained from shaper 122 and applied to a coincidence gate adapted to supply an editing pulse which is mixed with the control signal when the gate is opened by the vertical sync pulses from amplifier 114. Thus, when the sync pulses are in synchronism and timed relationship with the pulses generated by the photo-cell, an editing pulse is recorded to identify the end of a field, Fig. 5 (not shown). The phase comparator 119 comprises a bridge circuit comprising a number of resistors interconnected with four diodes having a common point. Opposite pairs of bridge terminals are respectively supplied from separate phase splitters, the error signal being taken from the common junction of the diodes, Fig. 4 (not shown). Specifications 798,927, 798,928, 798,929 and 798,930 are referred to.